Installation for nuclear magnetic resonance imaging

ABSTRACT

In a nuclear magnetic resonance imaging installation, monitors of the cathode tube type are shielded from the effect of an intense magnetic field. For this purpose, the installation according to the invention has a flat coil which creates a second magnetic field, the direction of which is opposite to that of the intense magnetic field.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to an installation for imaging by nuclearmagnetic resonance. The invention pertains especially to means which areused to shield instruments placed in the environment of the device thatproduces a magnetic field from the effect of the said magnetic field.This is the case, especially, for cathode screen type monitors used inthe operating consoles of devices for imaging by nuclear magneticresonance.

2. Description of the Prior Art

A nuclear magnetic resonance imaging device or an NMR device especiallycomprises a magnet which produces an intense, constant magnetic field inthe device to which the body of a patient to be examined is subjected.Such a device is for example depicted in ELECTROMEDICA, volume 51, n°2,1983, M. MORNEBURG: "GESICHTSPUNKTE BEI DER STANDORT SUCHE FUR EINMAGNETOM", page 67-72.

The NMR device is put into operation by an operator or operators who actat an operating console to define the various parameters and control thevarious operations needed to obtain the desired image. For this purpose,an operating console generally has two monitors: a first monitor calledan image monitor designed to display the image obtained and a secondmonitor called a dialogue monitor which, in particular, enables anoperator to adjust the different parameters. The use of the dialoguemonitor is made much easier when it is placed beneath a tactile screen.But it must be noted that the use of a tactile screen is not reallyworthwhile unless the dialogue monitor is itself a color monitor.

In general, the MRI device itself is placed in a first room which formsa Faraday cage. The operating console is placed in an adjoining room,and the wall between the two rooms has a glass window so that theoperators or doctors do not lose sight of the patient throughout thetime when he is placed in the MRI device, so that they can quickly be athis side if there is any hitch. For this reason, and also to reduce thearea of the premises needed, it is sought to place the operating consoleat a relatively small distance from the MRI device, for example, at lessthan 10 meters.

The problem that arises here relates to the use of the the monitors whenthey are subjected to the magnetic field produced by the MRI device.

For, when a monitor is set up in a magnetic field, it undergoesdisturbances which make it unusable. For example, with a black-and-whitemonitor placed in a three-Gauss magnetic field, it is impossible to usea tactile screen properly. With a color screen, the constraint is evengreater and the color screen is practically unusable when it is in afield of more than than 0.5 gauss.

The intense field produced by the MRI instrument can reach severalthousands of Gauss in the instrument itself, so that in the control nextto the examination room, the leakage field of the magnet, even at 10meters, is still far greater than the values referred to above.Consequently, it is noted that in the prior art, only black-and-whitemonitors can be used with the operating console at less than 10 metersfrom the MRI instrument, even then provided that these monitors arecontained in boxes forming shields. The use of a shield placed, forexample, around a monitor, not only makes it impossible to use a colormonitor at less than 10 meters from the NMR device but also has thedisadvantage of requiring a setting of the black-and-white instruments.For despite the use of shielding, this residual spurious field makes itnecessary to adjust the deflection coils proper to the monitor in orderto make it usable. This disadvantage is a particularly big one becausethe adjustment cannot be done unless the black-and-white monitor istaken out of its shield and the result of the adjustment is not visibleunless the monitor is again reintroduced into its shielding box. Thus,several successive adjusting operations are needed before the optimumsetting is obtained.

It is known by the EP-A No. 0 039 502 application a device forcorrecting the field effects on color monitor. Meanwhile this devicewhen it is set lead to the unsettlement of a similar device placed on aneighbouring monitor. As a result the entire correction is long to beachieved for the two monitors.

3. Summary of the Invention

A particular object of the present invention is to enable to the use ofat least one black-and-white or color monitor in a nuclear magneticresonance imaging insulation at a distance from the device which is farsmaller than in the prior art for the same field intensity, and toenable the use of the monitor or monitors in a far simpler and surer waythan in the prior art. It must be observed that, in substantially thesame conditions, the invention can be applied to the shielding of otherinstruments from the magnetic field, for example, magnetic tapes forwhich the use and storage near an MRI device may raise problems.

According to the invention, a nuclear magnetic resonance imaginginstallation comprises a device to produce an intense magnetic field,the said device being at a given distance from at least one instrumentthe operation of which may be disturbed by the intense magnetic field,the said installation comprising means to produce a second magneticfield in a direction substantially opposite to that of the intensemagnetic field, so that the second magnetic field tends to cancel thefirst intense magnetic field at the level of the instrument.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood from the following description,given as a non-exhaustive example, with reference to the two appendedfigures, of which:

FIG. 1 is a schematic view of a nuclear magnetic resonance imaginginstallation according to the invention;

FIG. 2 is a frontal view of a flat coil and an operating console shownin FIG. 1.

DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 gives a schematic view of a nuclear magnetic resonance imaginginstallation 1 or an MRI installation designed, in the non-exhaustiveexample described, for diagnostic use in medecine. According to astandard type of layout in hospital locations, the MRI installation 1 isdivided between two contiguous rooms, of which the first is anexamination room 2 and the second is a monitoring room 3. Theexamination room 2 contains the MRI device 4 proper, symbolized in FIG.1 by a rectangle set along a longitudinal axis Z. In practice, the MRIdevice 4 may have a generally circular cylindrical centered on thelongitudinal axis Z, with its interior part along the longitudinal axisZ designed to take a patient (not shown). The MRI device 4 comprises, ina manner which is conventional in itself, a coil or a magnet (not shown)that produces an intense, homogeneous and constant magnetic field B_(O)inside the MRI device 4, and especially in a volume under observationrepresented in FIG. 1 by a length under observation L having a center O.The direction of the field B_(O) inside the MRI device 4 is, forexample, the direction shown by the arrow 7, so that the MRI device 4may symbolize a magnet with one side 8, towards the monitoring room 3,forming a northern side N and the second side 9, opposite to theexamination room 3, forming a southern side S. The direction of thefield B_(O) is that of the longitudinal axis Z, and its intensity, whichis constant at all points of the length under observation L, is severalthousands of gauss along this length under observation L, for example,5000 gauss or 0.5 tesla.

The monitoring room 3 contains an operating console 10 through whichoperators (not shown) conduct the operations needed to obtain the imageof the patient. A partition 11, which separates the examination room 2from the monitoring room 3, has a communicating door 60 and a window 12through which the operators watch the patient throughout theexamination. According to a preferred layout, when permitted by theshape of the available premises, the plane of the window 12 issubstantially perpendicular to that of the longitudinal axis Z of theMRI device 4, and the operating console 10 is substantially parallel tothe window 12. This arrangement enables operators at front 13 of theoperating console 10 to handle the controls of the said operatingconsole 10 without losing sight of the patient.

As mentioned above, an operating console 10 has at least one monitor. Inthe non-exhaustive example described, the operating console 10 has afirst monitor and a second monitor 15, 16, set one on top of the other,so that the first monitor 15 which, in FIG. 1, is further in thebackground than the second monitor, is only partially shown. The screens17 of the monitors 15, 16, are pointed substantially towards the front13 of the operating console 10, and form a volume which is especiallysensitive to the intense magnetic field B_(O), this particularlysensitive volume being shown in FIG. 1 by a sensitive length L2 formedbetween the screen 17 and the rear 19 of the monitors 15, 16.

According to one characteristic of the invention, the MRI installation 1has means to produce a second magnetic field B_(c), which opposes thefirst field B_(O). In the non-exhaustive example described, the meansused to produce the second magnetic field B_(c), which opposes the firstmagnetic field B_(O), comprise a coil 20 of the flat coil type. The flatcoil 20 has a current (not shown) flowing through it in a direction suchthat a first side 21 of the flat coil 20 pointed to the MRI device 4constitutes a north pole N and its second side 22 constitutes a southpole S.

In the non-exhaustive example described, since the flat coil 20 iscentered on the longitudinal axis Z, the second magnetic field B_(c) isalso pointed along the axis Z but its direction on the axis Z, shown bythe second arrow 5, is opposite to the first direction 7 of the firstfield B_(O). The result of this is that, when adjusting the intensity ofthe second field B_(c), there is a tendency to compensate for and evento cancel the first field B_(O), especially on the sensitive length L2.

It must be noted that, within the spirit of the invention, the secondopposing magnetic field B_(c) may also be produced in a different way,for example, by a coil or coils (not shown) set in the monitors 15, 16themselves or, again, by a solenoid valve (not shown) of a great lengthwhich will be set around the monitors 15, 16 or even around theoperating console 10. Among these various means, the flat coil 20 has amajor advantage in that it occupies little space and, therefore, createshardly any inconvenience. For the flat coil 20 may have a circular orsquare section, and its plane may be parallel to that of the window 12,and it may even be applied to this window 12 as in the non-exhaustiveexample shown in FIG. 1. This position of the flat coil 20 is especiallyadvantageous in reducing the space occupied. However, the flat coil 20may have a different position with respect to the monitors 15, 16: forexample, it may surround the monitors 15, 16, or again, it may be set onthe other side of these monitors from the MRI device 4.

In the non-exhaustive example described, the monitors 15, 16, are setalong the longitudinal axis Z so that the second sensitive length L2 isidentical with or substantially parallel to the longitudinal axis Z. Theflat coil 20 is itself centered on the longitudinal axis Z and its planeis perpendicular to the latter. Thus, it is possible to virtuallynullify the magnetic field along the sensitive length L2, so thatblack-and-white or color monitors 15, 16 can be used without its beingnecessary to place these monitors 15, 16 in a shielding box as in theprior art.

For, assuming that the intense field B_(O) at the center O of the lengthunder observation L has a value of about 5000 gauss, its value on theaxis Z at a distance D of about 7 meters is about 10 gauss, i.e. thesensitive length L2 would be subjected to a magnetic field of 10 gaussin the absence of the flat coil 20, and this would make it impossible touse black-and-white type monitors 15, 16 without shielding and quiteimpossible to use color type monitors with or without shielding.

FIG. 2 shows a flat coil 20 and the operating console 10 seen from thefront of the said console 10, as shown in FIG. 1 by a third arrow 43.

FIG. 2 shows in the operating console 10 in the foreground, then theflat coil 20 in the background and, finally, further still in thebackground, the partition wall 11 which has the window 12. The operatingconsole 10 is raised above the floor 45 by a pedestal 46. The operatingconsole 10 has various conventional control and display elements (notshown) and has the first and second monitors 15, 16. The first monitor15 is the dialogue monitor which may comprise a tactile screen (notshown). The second monitor 16, placed above the first monitor, is theimage monitor. In the MRI equipment of the invention, these two monitors15, 16 may equally well be black-and-white or color type monitors. Theentire surface 47, presented by the two monitors 15, 16, parallel to theplane of the flat coil 20, is substantially centered on the center 26 ofthe flat coil 20. In the non-exhaustive example described, the center 26further represents the longitudinal axis Z of the MRI device 4 shown inFIG. 1.

The flat coil 20 is made of a conventional conductor forming turns 48(depicted in broken lines) mounted on a frame 31 made of a non-magneticmaterial such as wood for example. In the non-exhaustive exampledescribed, the section 39 of the flat coil 20, given by the shape of theframe 31, is substantially square. The sides 50 of the frame have athird length L3 of about 2 meters in the non-exhaustive exampledescribed, so that the mean distance a between the center 26 of the flatcoil 20 and its periphery represented by the frame 31 is about 1 meter,namely half the third length L3 of the sides 50.

Referring again to FIG. 1, the rear 8 of the desk 10 is preferably veryclose to the second side 22 of the flat coil 20, a few centimeters away,so that the rear of the monitors 15, 16 is also very close to it andmay, if necessary, penetrate the flat coil 20 so that, for example, itis at the level of the median plane 27 of the said flat coil. The flatcoil 20 has a thickness E of a few centimeters, for example 5centimeters. Since the monitors 15, 16, are generally always in the sameposition in operating consoles such as the operating console 10, thesaid operating console 10 and the flat coil 20 form a set which may bepositioned, with respect to the MRI device 4, differently to thearrangement shown in FIG. 1, so that, for example, it fits therequirements of existing sites. Thus, for example, the set formed by theflat coil 20 and the operating console 10 may be moved sideways along anaxis 25, crossing the longitudinal axis Z, so that it is suited tohospital configurations where the operating console 10 cannot be placedfacing the magnet or MRI device 4. However, this moving can be donewithin relatively small limits of about 0.6 meters on either side of thelongitudinal axis Z.

Nevertheless, the set formed by the flat coil 20 and the operatingconsole 10 can be moved to a far greater extent with respect to thelongitudinal axis Z, provided that this set is pointed in such a waythat the plane of the flat coil 20 is substantially perpendicular to thelocal direction of the magnetic field. This arrangement is shown in FIG.1 where the operating console 10 and the flat coil 20 are shown indashes and are respectively marked 10a and 20a, the flat coil 20a beingperpendicular to a second local direction U of the intense magneticfield B_(O).

In all cases, the value of the second magnetic field B_(c) can beadjusted to compensate for or even cancel the intense magnetic fieldB_(O) on the sensitive length L2, by adjusting the value of the currentwhich flows in the conductors (not shown) of the flat coil 20. This flatcoil 20 is supplied by an adjustable supply which is itself conventional(not shown).

According to one characteristic of the invention, the mean distance abetween the center 26 and the frame 31 is equal to or greater than thesensitive length L2 presented by the monitors 15, 16, so that the valueof the second field B_(c) set up by the flat coil 20 undergoesrelatively little variation on its axis. Furthermore, it must be notedthat, by placing the flat coil 20 between the MRI device 4 and themonitors 15, 16, the reduction, if any, in the value of the secondmagnetic field B_(c) along the sensitive length L2 is substantiallycompensated for by the reduction in the value of the intense field B_(O)in the direction of the axis Z as the distance from the MRI deviceincreases. For it is noted that, on the longitudinal axis Z, at adistance D of about 5.5 m. from the center O of the MRI device 4, theintense field B_(O) has a value of 18 gauss while, at 5.8 meters, theintense field B_(O) has a value of about 15 gauss. Consequently, byplacing the monitors 15, 16, so that the sensitive length L2 issubstantially opposite to the MRI device 4 with respect to the medianplane 26 of the flat coil 20, it is possible, with a flat coil 20 havinga radius or mean distance a equal to or greater than the sensitivelength L2, not only to obtain a situation where the opposing secondfield B_(c) varies to a fairly small extent, but also to obtaincompensation for this variation by the variation of the first intensemagnetic field B_(O).

It must be noted that, to a specialist in nuclear magnetic resonanceimaging, the method of the invention used to solve the problem of usingmonitors subjected to intense magnetic fields may cause surprise andeven disquiet because, to such a specialist, any magnetic field producedin the environment of an MRI device could be superimposed on the intensefield B_(O) and could destroy its homogeneity on the first length underobservation L.

However, computations have foreseen and tests have shown that the secondcompensation field B_(c) produced by the flat coil 20 at a relativelysmall distance D from the center O of the MRI device 4 gives only asmall modification to the value of the intense field B_(O) along thelength under observation L, and this modification in value causes only asmall displacement, during operation, of the central section withrespect to the original position, namely with respect to the center O.This modification of the value of the field B_(O) is eliminated duringthe calibrating operation which is conventional for an MRI device andwhich makes it possible to redefine the position of the central sectionwith respect to the original position. For, if it is assumed that thedistance D is 6.8 meters between the flat coil 20 and the center O ofthe length under observation L, and that the flat coil 20 has thefollowing characteristics given purely by way of example:

The frame 31 is a square the sides 50 of which have a length L3 equal to2a equal to 2 meters, giving a square of 2×2 m.;

The coil 20 has 100 turns through which flows a current of 16.5 amperes,giving 1650 amperes which make it possible to obtain the value of thesecond field B_(c) needed to compensate for the intense field B_(O)along the sensitive length L2; the flat coil 20 is centered on the axisZ, i.e. it is practically coaxial with the MRI device 4;

then the field B_(c) produced by the coil 20 on its axis Z at a distanceD is given by the following formula: ##EQU1## where μo is thepermeability of the vacuum=4π×10⁻⁷ ; I is the current in the coil 20; ais the mean distance between the center 26 of the coil and the frame 31;D is the distance from the center 26 to the flat coil 20.

It is found that for a distance D=0, namely at the center 26 of the coil20, the field B_(c) is equal 9.3 gauss.

It is found that for a distance D=6.8 meters, i.e. at the center O ofthe length under observation L, the value of the second compensationfield B_(c) is 0.04 gauss, which may have to be compensated for by thecalibration of the MRI device 4 as explained above. With respect to anymajor fault in the homogeneity of the field B_(O) measured, for example,along the longitudinal axis Z, which may be given by the secondcompensation field B_(c), its value is given by the derivative of thesecond field B_(c) on the axis Z, determined by the second formulabelow: ##EQU2## where μo=4π·10⁻⁷ ; I=1650 amperes; a is the meandistance equal to 1; D is equal to 6.8 meters and corresponds to thedistance between the center of the flat coil and the center of thelength under observation L; Tm=tesla per meter.

We get dBc/dB=17.10⁻⁷ T/m: this result shows that the field variation onone meter at the center O of the MRI device 4 is 0.017 gauss so that thefield gradient provided by the axis Z is smaller than one part permillion or one p.p.m. in a standard MRI device: it is hence smaller thanthe limit of measurability, and may therefore be neglected.

If the flat coil 20 is even closer to the center O of the MRI device 4,the field gradient or non-homogeneity given to the second field B_(c) onthe length under observation L may be greater than 1 p.p.m. In thiscase, this non-homogeneity can be easily compensated for by adjustingthe offset current of a gradient coil.

For it is known that MRI devices conventionally comprise, in addition toa coil producing the intense field B_(O), gradient coils with thefunction of creating a field gradient along three orthogonal axes, oneof which is the longitudinal axis Z. These gradient coils are generallymounted in a known way around the cylinder formed by the MRI device.Only one gradient coil 40, designed to act along the longitudinal axisZ, is represented in FIG. 1 for the greater clarity of this figure.

Field gradients are established only during an examination stage, sothat they are established according to a pulse. By continuously applyinga current, called an offset current, to one or more of these gradientcoils in a known way, the said offset current being capable of havingpulses superimposed on it, first order inhomogeneite of the intensefield B_(O) can be corrected by appropriately adjusting this current sothat each of the gradient coils may constitute a means to compensate fora non-homogeneity of the field B_(O) created in the MRI device 4 by theflat coil 20. Thus, returning to the non-exhaustive example described,where the flat coil 20 is perpendicular to the longitudinal axis Z andcentered on the said axis Z, it is possible to use the gradient coil 40and apply an offset current to it, making it possible to correct thefirst order homogeneity faults of the field B_(O) along the axis Z,including the faults created by the functioning of the flat coil 20.

This description is a non-exhaustive example showing how instruments canbe shielded from the effect of an intense magnetic field, especiallyinstruments of the type comprising a cathode tube. The invention isespecially valuable for MRI diagnostic installations, but can also beapplied for the shielding of instruments in all installations usingintense magnetic fields.

What is claimed is:
 1. A nuclear magnetic resonance imaging installationcomprising a device to produce an intense magnetic field, the saiddevice being placed at a given distance from at least one cathode tubemonitor to be shielded from the intense magnetic field, the saidmagnetic field having a first direction at the place of said monitor,the said installation comprising a flat coil to produce a secondmagnetic field having a second direction substantially opposite to thedirection of said first intense magnetic field, so as to tend to cancela leakage magnetic field produced by the intense magnetic field, at theplace of said monitor.
 2. An installation according to the claim 1wherein the monitor is of the color monitor type.
 3. An installationaccording to any of the claims 1 or 2 wherein, the monitor has a lengthsensitive to the magnetic field, the flat coil has a mean distancebetween its center and a peripheral part of this flat coil, and thismean distance is equal to or greater than the said sensitive length. 4.An installation according to any of the claims 1 or 2 wherein the planeof the flat coil is substantially perpendicular to a direction of theintense magnetic field at the place of said monitor.
 5. An installationaccording to any of the claims 1 or 2 wherein the monitor issubstantially centered on an axis of the flat coil said axis of saidflat coil being an axis perpendicular to the plane of said flat coil atthe center of said flat coil.
 6. An installation according to the claim3 wherein the flat coil is placed substantially between the device andthe monitor at a distance approximately egal to the sensitive length. 7.An installation according to any of the claims 1 or 2 wherein the flatcoil has substantially square section.
 8. An installation according tothe claim 4 wherein a first direction of the intense magnetic field isidentical with the longitudinal axis of the device and wherein thecenter of the flat coil is substantially placed on the said longitudinalaxis.
 9. An installation according to any of the claims 1 or 2 furthercomprising means to compensate for a non-homogeneity of the intensefield created in the device by the second magnetic field.